The present invention relates to hull adjuncts for water craft, more particularly hull adjuncts which improve hydronautics or enhance utility or safety for planing water craft such as outboard motor boats and other power craft.
Many water craft, notably power craft, utilize hulls which are designed to hydroplane ("plane") upon attainment of sufficient speeds to overcome gravity-dominated water fluid flow regimes. While in the planing mode of operation (i.e., at higher speeds) the water craft operates on the water surface with most of the lift force provided by hydrodynamic rather than bouyant effects. In the subplaning mode of operation (i.e., at lower speeds) lift is provided for the craft by combined buoyant and hydrodynamic forces; in this regime drag forces are high and large amounts of energy are ultimately expended in the bow wave and wake.
Conventional planing craft have hulls which are designed for efficiency and stability in the planing mode but which are less efficient in the subplaning mode; indeed, many hulls designed for efficient planing operation are markedly inefficient at lower speeds.
A relevant hull design criterion with regard to performance of planing hulls is the length-to-beam ratio (i.e., L.sub.H /B.sub.H, wherein L.sub.H =length and B.sub.H =width or "beam"), which is the ratio of the hull length L.sub.H to the hull width B.sub.H. Generally speaking, hull efficiency at subplaning speeds increases with increasing L.sub.H /B.sub.H ; and, conversely, hull efficiency at planing speeds decreases with increasing L.sub.H /B.sub.H.
An instructive study which was published in 1964 by the Society of Naval Architects and Marine Engineers explored various parameters affecting performance of planing hulls; these parameters included length-to-beam ratio, relationship between hull size and gross weight, and location of the longitudinal center of gravity ("LCG"). See Clement, E. P., and Blount, D. L., "Resistance Tests of a Systematic Series of Planing Hull Forms," presented at the Annual Meeting of the Society of Naval Architects and Marine Engineers (Nov. 14 and 15, 1963), New York, 1964. These model tests demonstrated, inter alia, that an increase in hull length of about 8 percent generally results in an overall reduction in resistance of about 20 percent, and that transition from the low speed mode to planing operation occurs at Froude numbers of about 1.5.
Froude number F.sub..gradient. is defined as ##EQU1## where v is speed in feet per second, g is acceleration due to gravity, and .gradient. is volume of displacement at rest. At F.sub..gradient. =1.5, drag normally is near peak value, and speed increases beyond that transition value typically bring about a reduction of hull drag until a minimum value occurs at about F.sub..gradient. =2.0 or beyond.
The following curve fit expression may be developed from the Clement-Blount data and considered to mathematically represent the general behavior of modern planing hulls in terms of drag-to-weight ratio D and the above-noted three parameters: EQU D=(1.81.times.10.sup.-7)(20.8-y).sup.2 (80.9+z).sup.2 (x).sup.-0.68( 2)
where x is the length-to-beam ratio L/B; y is the ratio A/.gradient..sup.2/3, wherein A is the planing bottom area; and z is the distance, expressed as a percentage of length L, of the longitudinal center of gravity ("LCG") from the centroid of area A. Values for x generally range between 2.0 and 6.0 inclusive, for y between 5.5 and 10.0 inclusive, and for z between 4.0 and 12.0 inclusive.
Drag-speed equations and power-speed equations may be derived from Eq. (2) as follows: EQU D.perspectiveto.K.sub.d v.sup.2 /w.sub.o ( 3) EQU and EQU P.perspectiveto.K.sub.p v.sup.3 /w.sub.o ( 4)
where D is the drag-to-weight ratio, P is the power-to-weight ratio, K.sub.d is the drag constant, K.sub.p is the power constant, v is the speed of the water craft in feet per second, and w.sub.o is its weight.
L.sub.H /B.sub.H values for planing craft hulls are typically between about 2.5 and 5.0, with many such hulls designed using L.sub.H /B.sub.H values in the 2.8 to 3.3 range. These hull designs are efficient at planing speeds but are wasteful of energy at subplaning speeds. Increasing the L.sub.H /B.sub.H value for a given craft hull by increasing hull length l and/or decreasing hull width b will increase hull efficiency at subplaning speeds, in some cases significantly so. The aforenoted published series tests of Clement and Blount demonstrated, for example, that an 8 percent increase in hull length for a typical planing hull design having an L.sub.H /B.sub.H of 3.0 and a specific loading coefficient of 8.5 results in an overall reduction in resistance of about 20 percent during the subplaning mode of operation. However, increasing the L.sub.H /B.sub.H value thusly will decrease hull efficiency at planing speeds, since more power will generally be needed for a craft with a higher L.sub.H /B.sub.H -ratio hull than for a craft with a lower L.sub.H /B.sub.H -ratio hull, in order to travel at like planing speeds.
In addition to the aforementioned considerations regarding hull efficiency, typical planing water craft may be lacking in other areas, as well. Users of such craft often require or desire additional deck space in order to enable or facilitate activities such as fishing, swimming and diving. Moreover, navigational safety is of paramount concern for boatmen and passengers and other seafarers and mariners. A specific safety need for a typical planing craft is means of protection from or mitigation of craft damage or personal injury due to collision with natural or architectural formations or structures or with other water craft.